1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine, and more particularly, to a control apparatus for an internal combustion engine for estimating a throttle upstream pressure, which being a pressure between a compressor and a throttle in an internal combustion engine equipped with a supercharger.
2. Description of the Related Art
Hitherto, there has been known a control system for an internal combustion engine (hereinafter referred to as “engine”) equipped with a supercharger in order to increase an output of the engine. Known examples of the supercharger include a turbocharger (hereinafter also referred to as “T/C”) in which a compressor to be driven by a high-speed rotation of a turbine rotated by energy contained in an exhaust gas is installed in an intake system of the engine, and a mechanical supercharger (hereinafter also referred to as “S/C”) in which a compressor to be driven by a crankshaft via a belt is installed in the intake system of the engine.
Moreover, as a method of detecting an intake air amount in the engine control system, there have hitherto been known a method (hereinafter referred to as “AFS method”) of detecting the intake air amount by using an airflow sensor (hereinafter referred to as “AFS”), and a so-called speed density method (hereinafter referred to as “S/D method”) for estimating the intake air amount based on a pressure in an intake manifold (hereinafter referred to as “intake manifold pressure”). The AFS method directly measures an air flow rate (hereinafter referred to as “AFS intake air amount”) passing through an installed location of the AFS (hereinafter referred to as “AFS portion”). An air flow rate (hereinafter referred to as “cylinder intake air amount”) taken into a cylinder in a steady state operation is approximately equal to the AFS intake air amount. Accordingly, the AFS method has such a feature that a calculation error of the cylinder intake air amount in the steady state operation is relatively small compared with the S/D method. However, a distance from the AFS portion to the cylinder is long in the control system for an engine equipped with a supercharger, and hence a response lag and a density change need to be simulated for this distance to calculate the cylinder intake air amount. Thus, a calculation accuracy of the cylinder intake air amount in a transient operation is considered to depend on an accuracy of a physical model of the intake system for simulating the response lag and the density change. Then, a physical model of the intake system for dealing with this problem has hitherto been proposed (for example, refer to Japanese Patent Application No. 2014-084647 (hereinafter referred to as Prior Application 1)).
In a control system for an engine equipped with a turbocharger, which being a first embodiment of Prior Application 1, there is disclosed a method in which: an airflow sensor is used as a flow rate sensor; an atmospheric pressure sensor, a throttle upstream pressure sensor, and an intake manifold pressure sensor are used as pressure sensors; and a cylinder intake air amount and a throttle intake air amount are calculated based on information from those sensors. In a control system for an engine equipped with a mechanical supercharger, which being a second embodiment of Prior Application 1, there is disclosed a method in which: an airflow sensor is used as a flow rate sensor; an atmospheric pressure sensor, an S/C upstream pressure sensor, and an intake manifold pressure sensor are used as pressure sensors; and the cylinder intake air amount is calculated based on information from those sensors. However, there is a problem in that the use of such a large number of sensors leads to an increase in cost. Thus, as a method of estimating a physical amount without using any sensor, for example, there has been proposed a method of estimating the atmospheric pressure in a control system for an engine without a supercharger (hereinafter referred to as “N/A system”) (for example, refer to Japanese Patent Application No. 2014-023903 (hereinafter referred to as Prior Application 2)).
In Prior Application 2, so as to precisely realize so-called torque-based control in the N/A system, there is disclosed a method of precisely estimating the atmospheric pressure in a wide operation region while carrying out throttle opening degree learning control for absorbing a machine difference dispersion of a throttle, thereby precisely controlling the intake air amount by performing throttle control while the atmospheric pressure sensor is omitted.
As described above, the control system for an engine equipped with a mechanical supercharger, which is the second embodiment of Prior Application 1, is configured so that the airflow sensor is used as the flow rate sensor, the atmospheric pressure sensor, the S/C upstream pressure sensor, and the intake manifold pressure sensor are used as the pressure sensors, and the cylinder intake air amount is calculated based on the information from those sensors, and hence a large number of sensors need to be used, resulting in a problem in that the cost is increased.
It is conceivable to apply the method of Prior Application 2 to eliminate the atmospheric pressure sensor in the case where the pressure on the upstream side of the throttle is the atmospheric pressure as in the second embodiment of Prior Application 1. However, the pressure on the upstream side of the throttle is not the atmospheric pressure in the first embodiment of Prior Application 1, and the atmospheric pressure cannot be estimated. In this case, even when an attempt is made to estimate the throttle upstream pressure, the throttle upstream pressure significantly changes depending on the operation state, and hence it is considered that it is difficult for the method of Prior Application 2 alone to follow the speed of the change, resulting in a problem in that the throttle upstream pressure cannot be precisely estimated.